From what I understand, when a chemical reaction within the battery occurs, the zinc becomes negatively charged and the copper rod becomes positively charged. When an electron flows from "+" to "-", it is resisted by the electric field by the negative terminal. The chemical reaction does "work" (that word makes me cringe) on the charges to move it to the negative terminal. That amount of work per unit of charge is voltage. I'm a physics newbie, so please bear with me!